1. Field of the Invention
The present invention relates to an image comparison apparatus and image comparison method which compare a reference image as a sample with a comparison image as a comparison target, and a program for causing a computer to execute image comparison.
2. Description of the Related Art
Recently, many troubles have been caused by counterfeiting of printed matter, counterfeiting of special printing/treatment on ID cards, and the like. Counterfeiting methods associated with such troubles are so sophisticated as to make identification of counterfeits difficult. In addition, printing or treatment for preventing counterfeiting becomes too elaborate to be checked by the human eye.
Under the circumstances, a comparison microscope like the one shown in FIG. 1 has recently been proposed.
This comparison microscope includes two microscope bodies 202a and 202b. The microscope bodies 202a and 202b respectively have object lenses 203a and 203b. Specimens 204a ad 204b such as printed matter are placed on stages 205a and 205b corresponding to the object lenses 203a and 203b. A common comparison lens barrel 201 is connected to the microscope bodies 202a and 202b. This arrangement allows simultaneous observation of observation images formed by the microscope bodies 202a and 202b through an observation lens barrel 200. For example, the specimen 204a serving as a sample is placed on the stage 205a of the microscope body 202a. The specimen 204b serving as a comparison target is placed on the stage 205b of the microscope body 202b. An observation image of the right half of the specimen 204a in the visual field is abutted against an observation image of the left half of the specimen 204b in the visual field to allow observation of these images as one observation image through the observation lens barrel 200. This makes it possible to compare the left and right observation images and allow the user to easily check whether or not the specimens 204a and 204b are identical.
An apparatus using comparison microscopes like those described above requires a real specimen serving as a sample as well as a specimen serving as a comparison target for which counterfeit identification is required. Therefore, an observer who has no specimen as a sample cannot perform comparing operation, and has a difficulty in counterfeit identification. In addition, since the two microscope bodies 202a and 202b are used, variations in brightness and color between the optical systems of the microscope bodies 202a and 202b sometimes make it difficult to compare a comparison image with a reference image. It is important for counterfeit identification to facilitate recognition of the difference between images by positioning them for comparison. Such positioning, however, requires cumbersome operation, e.g., moving the positions of the specimens 204a and 204b on the stages 205a and 205b. 
On the other hand, an image comparison apparatus for performing comparison and observation using reference images stored as digital images is known (see Jpn. Pat. Appln. KOKAI Publication No. 06-6807). In this image comparison apparatus, a plurality of images captured from a TV camera are stored as digital data. Subsequently, a half portion of a reference image and a half portion of a comparison image are extracted and pasted together to display them as one image on a monitor. This makes it possible to easily check whether or not the left and right images are identical.
In the above image comparison apparatus, recorded digital image are read out and compared with each other on the TV monitor. Since still images that have already captured (stored) are juxtaposed, a desired position on a reference image for comparison is difficult to adjust to a comparison image. In addition, since images are simply arranged side by side, it is sometimes difficult to observe the differences between them. When a reference image and comparison image are to be compared with each other, the same photographing conditions, e.g., a magnification, position, and brightness, are preferably set for these images. When image files obtained by photography in the past are used for comparison, photographing conditions cannot often be known. This makes it difficult to accurately compare and observe images. Assume that an observation window is split into two parts and two images are displayed therein to be compared in order to prevent a decrease in resolution. In this case, half images must be compared with each other, and hence comparison/observation of images must be performed at least twice. In addition, when an observation window is split into two parts and images are displayed therein, the observer may be confused about which image is a reference image or comparison image. Furthermore, since differences between two images are visually checked by the observer, an oversight and the like may occur.